.loginbox{
    position: fixed;
    width: 455px;
    top: 50%;
    left: 50%;
    margin-left: -227px;
    margin-top: -164px;
}
.loginbox>div{
    background-color: rgba(255,255,255,0.6);
    border-radius: 4px;
    box-shadow: 5px 5px 20px #4444;
    padding: 20px;
}